Feature: allow acoustic domain and pressure seismograms (specfem2d)

[bch0w]

Related to #151

SeisFlows was not completely tooled to deal with pressure seismograms and acoustic domains. Especially in SPECFEM2D when seismogram file naming can change enough to fail wildcard matching under certain conditions. This PR improves SeisFlows support for dealing with acoustic domains and pressure seismograms.

Changelog:

• Parameter components (used by: solver.specfem and preprocess.pyaflowa) now defaults to NoneType. Previously it was set as 'ZNE'. This makes the default behavior of SeisFlows wildcard searing more general/forgiving
• Made the solver.specfem.data_wildcard() function wildcard search for seismograms much more general to be able to catch all the different types of ASCII and SU filenames that SPECFEM2D might generate
• Default preprocessing module now allows pressure seismograms, requiring that unit_output='PRE'. Before, pressure seismograms were not an acceptable unit output.

To use acoustic domain and pressure seismogram a User will first need to set up their SPECFEM2D Par_file:

seismotype = 4

and their SeisFlows YAML file:

materials: acoustic
unit_output: pre
components: null